The present invention relates to a circuit for efficiently driving a stepping motor.
Stepping motors are suitable for digitally controlled machines, such as X-Y plotters, or the like. When driving these machines using stepping motors, it is desirable that the rotors of these stepping motors be efficiently and correctly rotated. The coils, or windings, associated with the stepping motors present an inductive load to an applied signal. As a result, each coil may be considered a series combination of an inductor and a resistor, thus, they may have associated therewith undesirably long time constants. Moreover, counter electromotive forces may be generated in the coils during rotation of the rotor. To excite the stepping motor by applying a drive voltage to its coils that voltage must overcome the effects of the inductance and resistance associated with the coils. The inductance effect impedes the establishment of, or a change in, the current flowing in the coil. As the switching speed of the drive voltage increases, the rise time of the current to the desired operation level becomes more significant and the torque exerted on the rotor becomes weaker. Thus, it is well known that it is necessary to shorten the rise time of the current in order to maintain the coordination of the stepping motor in a high speed environment. To this end, various conventional techniques have been widely used.
One of the conventional techniques involves the insertion of a resistor in series with the coil of the stepping motor and raising the power supply voltage in order to decrease the time constant thereof. However, this technique has a disadvantage in that power is wasted as a result of the inserted resistor. Another conventional technique involves the use of two power supplies, wherein a high voltage from one power supply is applied to the coil of the stepping motor for shortening the current's rise time at a certain switching time, and a low voltage from the other power supply is applied to the coil after the current in the coil reaches a predetermined value. This conventional technique is complex and the means for accomplishing this conventional technique is expensive in construction as a result of the need for two power supply circuits and a power voltage switching circuit.
A further conventional scheme involves the use of a current regulator for controlling the flow of current through the coil. However, this further conventional scheme is also complex and expensive to accomplish.